P14-light And Lenses Flashcards
What is the focal length of a lens
The focal length of a lens is the distance from the centre of the lens to the principle focus
What is the principle focus of a lens
The principle focus of a lens is the the point where all the rays appear to focus
Why are the central rays passing through the centre of the lens not refracted ?
The central rays passing through the centre of the lens are not refracted as they pass along the normal (the principle axis
What is a real image
A real image is an image where all the rays meet at a point
-An object will form a real image if it is placed behind the fist focal length
What happens to an object that is placed between the first and second focal length
If an object is placed between the first and second focal length it forms a real image that is inverted and magnified (bigger)
What happens if an object is placed after the second focal length
If an object is placed after the second focal length it still forms a real image that is inverted but the image is smaller than the object (diminished)
What does a concave lens to do light rays
Concave lenses diverge light rays
Do light rays come from the principle focus in concave lenses
In concave lenses the light rays only appear to be coming from the principle focus they do not actually come from the principle focus
what are the features of an image for a concave lens
For a concave lens the image is diminished (smaller than the real object) , virtual ( as the lines do not actually cross there) and the right way up
What are the steps of drawing a ray diagram for the convex lens
- First draw a line from the top of the object through the centre of the lens without changing direction
- Then draw a line from the top of the object parralel to the principle axis until it reaches the lens. When it reaches the lens it should refract
- Draw a dotted line from the first principle focus to the refracted ray
- The point where these two lines meet is where the image forms
What are the steps of drawing a ray diagram for a concave lens
First draw a line from the top of the object through the centre of the lens that carries on in a straight line
Then draw a line that runs parallel to the principle axis until the lens
Then finally draw a dotted line that runs frp, the principle focus on the side of the object until it reaches the line that runs parralel to the principle fovus
Where the dotted line and line running through the centre of the lens meet is the top of the object(it will be diminished , virtual and on the same side as the object)
What happens if an object is placed at 2F
If an object is placed at 2F it will form a real image the same size as the object
What happens if an image is placed between the centre of the lens and the focal length
If an object is placed between the first focal length and the centre of the len,s the rays will never meet so you have to trace backwards in order to find an image
-The image will be magnified , virtual and the right way up for an image between the centre of the lens and the first focal point
How do you draw a ray diagram if the object is between the centre of lens and the first focal point
If the object is between the first focal length and the centre of lens you 1. Draw a ray from the top of the object through the centre of the lens carrying on straight
2 .Draw a line from the top of the object parralel to the principle axis until it reaches the lens
3. Extend the rays back with dotted lines until the dotted lines meet. Where the dotted lines meet shows the position of the image
Note: Extend the ray parralel to the principle focus back from where it meets the lens not from the top of the object
When is the only time a convex lens produces a virtual image
The only time a convex lens produces a virtual image is when an object is placed between the centre of the lens and the first focal point
-When an object is placed here it can be used for a magnifying glass
